Thymuses of 14-day-old AKR mouse embryos were infected with Gross murine leukemia virus (MuLV) and then maintained in organ culture for 3 weeks. When they were transplanted to 3-week-old (AKR X C3H)F1 mice, approximately 50% of these developed T lymphomas within 3-4 months. Most (22/23) tumors were of host, F1-hybrid, origin while only one was of donor AKR type. No clear evidence for in vitro MuLV-induced lymphoma cells was therefore obtained. Exposure of MuLV-infected embryonic thymuses to interferon during the organ culture period significantly reduced the incidence of lymphomas in mice receiving such thymus transpalnts...